Initially, as a part of the kraft pulping process for making paper products, wood chips are converted to pulp by cooking the wood chips with white liquor in digesters at elevated temperature and pressure. White liquor is a water solution comprising sodium sulfide and sodium hydroxide. The white liquor dissolves the lignin that binds the cellulose fibers of the wood chips together. When cooking is complete, the contents of the digesters are sent to the pulp washers to separate the pulp from the spent cooking liquor. The separated pulp then proceeds through numerous stages until it becomes the finished paper product.
The combination of the spent cooking liquor and pulp wash water is referred to as weak black liquor. The weak black liquor is concentrated to form black liquor with a solids content of approximately 65 percent. Black liquor comprises lignin, sodium carbonate, sodium sulfate, and other inorganic salts. The weak black liquor is sent to the multiple-effect evaporator system where the black liquor is concentrated to about 55 percent solids. The black liquor is then fired in a recovery boiler to generate useful steam for the paper mill and to allow for the recovery and recycle of the cooking chemicals as green liquor. The resultant green liquor is converted back to white liquor for return to the digesters. The recovery of these costly cooking chemicals is essential to the economic operation of a paper mill. Lignin is a low value waste by-product from the papermaking industry.
The use of lignin as a renewable carbon feedstock for the synthesis of biofuels is a potential high value outlet for lignin. However, lignin is not directly useful as a refinery feedstock for biofuels synthesis. First, solid materials such as lignin are difficult to handle in conventional refinery systems that typically process liquid streams. Second, the high oxidation state of lignin (by virtue of its phenolic characteristics) means it has a low energy content compared to common refinery fuels. Third, lignin is so highly oxygenated that it is not generally compatible with hydrocarbon streams.
For these reasons, a process to generate a refinery-compatible hydrocarbon feedstock from lignin is needed. The present invention discloses a process for generating a biofuels feedstock from lignin.